Organic light emitting diodes (OLED), also known as organic electroluminescent (EL) devices, are a class of electronic devices that emit light in response to an electrical current applied to the device. The structure of an OLED device generally includes an anode, an organic EL medium, and a cathode. The term, organic EL medium, herein refers to organic materials or layers of organic materials disposed between the anode and the cathode in the OLED device. The organic EL medium can include low molecular weight compounds, high molecular weight polymers, oligimers of low molecular weight compounds, or biomaterials, in the form of a thin film or a bulk solid. The medium can be amorphous or crystalline. Organic electroluminescent media of various structures have been described in the prior art. Dresner, in RCA Review, 30, 322 (1969), described a medium comprising a single layer of anthracene film. Tang et al., in Applied Physics Letters, 51, 913 (1987), Journal of Applied Physics, 65, 3610 (1989), and commonly assigned U.S. Pat. No. 4,769,292, reported an EL medium with a multi-layer structure of organic thin films, and demonstrated highly efficient OLED devices using such a medium. In some OLED device structures the multi-layer EL medium includes a hole-transport layer adjacent to the anode, an electron-transport layer adjacent to the cathode, and disposed in between these two layers, a luminescent layer. Furthermore, in some preferred device structures, the luminescent layer is constructed of a doped organic film comprising an organic material as the host and a small concentration of a fluorescent compound as the dopant. Improvements in EL efficiency, chromaticity, and stability have been obtained in these doped OLED devices by selecting an appropriate dopant-host composition. The dopant, being the dominant emissive center, is selected to produce the desirable EL colors. Examples of the doped luminescent layer reported by Tang et al. in commonly assigned U.S. Pat. No. 4,769,292 and by Chen et al. in commonly assigned U.S. Pat. No. 5,908,581 are: tris(8-quinolinol)-aluminum (AlQ) host doped with coumarin dyes for green emitting OLEDs; and AlQ doped with 4-dicyanomethylene-4H-pyrans (DCMs) for orange-red emitting OLEDs. Shi et al., in commonly assigned U.S. Pat. No. 5,593,788, disclosed that a long operational life was obtained in an OLED device by using a quinacridone compound as the dopant in an AlQ host. Bryan et al., in commonly assigned U.S. Pat. No. 5,141,671, disclosed a luminescent layer containing perylene or a perylene derivative as a dopant in a blue emitting host. They showed that a blue emitting OLED device with an improved operational stability was obtained. In both disclosures, the incorporation of selected fluorescent dopants in the luminescent layer is found to improve substantially the overall OLED device performance parameters. Co-doping of luminescent layer with anthracene derivatives results in devices with better stability as shown in JP 99273861 and JP 284050. Doping the hole-transport layer with materials that impede hole-transport and co-doping hole-transport materials into electron-transporting AlQ leads to the improved device lifetimes, Popovic et al. Thin Solid Films 2000, 363, 6; SPIE 1998, 3476, 68.
The most common formulation of the doped luminescent layer includes only a single dopant in a host matrix. However, in a few instances, incorporation of more than one dopant in the luminescent layer was found to be beneficial in improving stability. Using a luminescent layer containing rubrene, a yellow emitting dopant, and DCJ, 4-(dicyanomethylene)-2-methyl-6-[2-(4-julolidyl)ethenyl]-4H-pyran, a red emitting dopant, in an AlQ host, it is possible to produce a red emitting OLED device with improved operational stability, Hamada et al. in Applied Phys. Lett. 75, 1682 (1999); EP 1162674. Here rubrene functions as a co-dopant in mediating energy transfer from the AlQ host to the DCJ emitter. Generally, in dual dopant systems, it has been noted that the operational stability tends to increase compared to that of the single dopant systems.
Although EL efficiency, color, and stability have been improved significantly using doped luminescent layers of various compositions, the problem of low operational stability persists. Insufficient stability presents the greatest obstacle for many desirable practical applications.